Transmission electron microscopes (TEMs) allow observers to see extremely small features, on the order of nanometers. In contrast to scanning electron microscopes (SEMs), which only image the surface of a material, TEMs also allow analysis of the internal structure of a sample. In a TEM, a broad beam impacts the sample and electrons that are transmitted through the sample are focused to form an image of the sample. The sample must be sufficiently thin to allow many of the electrons in the primary beam to travel though the sample and exit on the opposite side. Samples, also referred to as lamellae, are typically less than 100 nm thick.
In a scanning transmission electron microscope (STEM), a primary electron beam is focused to a fine spot, and the spot is scanned across the sample surface. Electrons that are transmitted through the work piece are collected by an electron detector on the far side of the sample, and the intensity of each point on the image corresponds to the number of electrons collected as the primary beam impacts a corresponding point on the surface. The term “TEM” as used herein refers to a TEM or a STEM and references to preparing a sample for a TEM are to be understood to also include preparing a sample for viewing on a STEM. The term “S/TEM” as used herein also refers to both TEM and STEM.
Focused Ion Beam (FIB) microscope systems produce a narrow, focused beam of charged particles, and scan this beam across a specimen in a raster fashion, similar to a cathode ray tube. Unlike the SEM, whose charged particles are negatively charged electrons, FIB systems use charged atoms, hereinafter referred to as ions, to produce their beams. These ions are, in general, positively charged.
Removing material from a substrate to form microscopic or nanoscopic structures is referred to as micromachining, milling, or etching. When an ion beam is directed onto a semiconductor sample, it will eject secondary electrons, secondary ions (i+ or i−), and neutral molecules and atoms from the exposed surface of the sample. By moving the beam across the sample and controlling various beam parameters such as beam current, spot size, pixel spacing, and dwell time, the FIB can be operated as an “atomic scale milling machine,” for selectively removing materials wherever the beam is placed. The dose, or amount of ions striking the sample surface, is generally a function of the beam current, duration of scan, and the area scanned. The ejected particles can be sensed by detectors, and then by correlating this sensed data with the known beam position as the incident beam interacts with the sample, an image can be produced and displayed for the operator. Determining when to stop processing is referred to as “endpointing.” While there are several known methods for detecting when a micromachining process cuts through a first material to expose a second material, it is typical to stop laser processing before a change in material is reached, and so determining the end point is more difficult.
FIB systems are used to perform microsurgery operations for executing design verification or to troubleshoot failed designs. This usually involves “cutting” metal lines or selectively depositing metallic lines for shorting conductors together. FIB “rapid prototyping” is frequently referred to as “FIB device modification”, “circuit editing” or “microsurgery.” Due to its speed and usefulness, FIB microsurgery has become crucial to achieving the rapid time-to-market targets required in the competitive semiconductor industry.
Successful use of this tool relies on the precise control of the milling process. Current integrated circuits have multiple alternating layers of conducting material and insulating dielectrics, with many layers containing patterned areas. The milling rate and effects of ion beam milling can vary vastly across the device. This is the reason why endpointing is difficult to perform without it being destructive. Endpointing is generally done based on operator assessment of image or graphical information displayed on a user interface display of the FIB system. In most device modification operations, it is preferable to halt the milling process as soon as a particular layer is exposed. Imprecise endpointing can lead to erroneous analysis of the modified device.
As semiconductor device features continue to decrease in size from sub-micron to below 100 nm, it has become necessary to mill smaller and higher aspect ratios with ion beam currents. FIB operators rely on conventional methods using real-time images of the area being milled and a graphical data plot in real time, to determine proper endpoint detection. Generally, the FIB operator is visually looking for changes in brightness of the milled area to qualitatively determine endpoint detection. Such changes may indicate a transition of the mill through different materials, such as a metal/oxide interface. The operator uses the progression slice to slice and looks for changes that ultimately tell the operator where the milling is taking place, the changes in the sample, and the progression towards endpointing.
Modern techniques sometimes involve the use of dual beam systems, such as an a FIB and SEM combination systems that allow the user to slice through samples and create images of the sections “live,” such as SPI—(simultaneous patterning and imaging mode), for real-time imaging feedback on the milling processes. TEM sample prep endpointing is a decision made in real time and it can be used in cross section patterns, but the sample is generally sliced in a manner that is also destructive. In addition, SPI images often create lower resolution due to the frame averaging of the images and the high image e-beam currents. The I-SPI is a system that allows images to update between various slices. These images are refreshed at every slice, but because consecutive slices involve only slight changes between images, the user often finds these image slices very difficult to follow.
There are generally two different ways to collect a stack of 2D SEM images of FIB milled surfaces for subsequent 3D modeling of volumes using a dual platform FIB/SEM instrument, i.e., in static or dynamic mode. In dynamic SEM imaging of FIB milled surfaces (i.e., SPI mode), SEM images are acquired in real time during the FIB milling process. In static image acquisition mode, the FIB is used to slice away material and then either paused or stopped so that a slow scan high resolution SEM image may be acquired. This type of image acquisition can be easily programmed into an automated Slice and View algorithm or an intermittent or I-SPI mode of instrument operation.
In SPI mode, secondary electrons (SEs) are emitted and detected due to ion/solid interactions as well as electron/solid interactions. To swamp out the SE signal from the FIB milling in the SEM image, the SEM image acquisition must be performed by changing three critical SEM imaging parameters: (i) the SEM beam current must operate at approximately a factor of 2 or greater than the ion beam milling current, (ii) the SEM images must be acquired at very fast scan rates, and (iii) the SEM images must be acquired using frame averaging (e.g., as many as 32 or 64 frames may be required for large beam current milling). Thus, SE SEM acquisition of images obtained in SPI mode must be collected in a mode which is typically not used for highest resolution imaging. Alternatively, backscattered electron (BSE) SEM images can be collected in SPI mode where the SEs from the FIB milling produces negligible artifacts in the BSE imaging process. However, the timing of image acquisition during SPI mode is critical, and even the acquisition of BSE SEM images in SPI mode may be non-trivial.
SEM images acquired in SPI mode can obtain redundant and/or duplicate information from one or more slices. Thus, using SPI mode, one would have to manually search through the sequence of images to remove redundant images such that an accurate 3D model could be constructed. One could time each SE or BSE image saved such that it occurs only after a complete FIB slice, but this would require a prior knowledge of the material sputtering characteristics and would be difficult to exactly correlate the SEM acquisition time with the time needed to FIB through a slice. It is noted however that SPI mode is extremely useful for endpointing any FIB operation since FIB milling may be monitored in real time.
The advantage to the static Slice and View methods for 3D modeling is that a high resolution slow scan SEM image is acquired after each FIB milled slice is completed. Thus, each image corresponds uniquely to each FIB slice for easy volume determination. In addition, automated SEM beam shift and auto-focus corrections can be implemented to keep the region of interest centered and focused as sectioning progresses.
Prior art methods have tried to improve on FIB milling endpointing operations by generally creating a real-time ability to gauge the sensitivity to regions of interest on a sample site. For example, EP 1812946 A1, with a filing date of Nov. 15, 2005 (also published as U.S. Pat. No. 7,897,918), titled “System and method for focused ion beam data,” (hereinafter as the '918 patent) discloses a system and method for improving FIB milling endpointing operations by using real-time graphical plots of pixel intensities with an increased sensitivity over native FIB system generated images and plots. This is done by receiving dwell point intensity values and creating raster pattern data to create areas of sensitivity. As shown in FIG. 9 of the '918 patent disclosed a method of using snapshot images taken progressively in a raster pattern. The frame generation is done using a CPB system. And as shown, the differential images 428, 430 are made using the individual slices.
This method has many shortcomings, which should be apparent. The accuracy and timelines of this procedure does not allow the operator full control of the endpointing nor would it allow the operator to see clear distinctions in patterns or defects. Further, the time consumption for this procedure would not allow the operator to perform this function in a manner that can enable real-time endpointing with concurrent use of progressive endpointing.
With current technologies that can be used to manipulate endpointing, there are generally two modes for collecting a series of SEM images of FIB-milled surfaces using a dual beam instrument: static mode and dynamic mode. In dynamic SEM imaging of FIB milled surfaces, SEM images are acquired in real time during the FIB milling process. In static image acquisition mode, the FIB is used to slice away material and then either paused or stopped so that a slow scan high resolution SEM image may be acquired.
One challenge during dynamic mode imaging, is to know exactly where the beam is hitting the sample, and what part of the sample is being milled at any given moment. Particularly in the preparation of a TEM sample, the user must determine in real time when to stop thinning a sample. Thinning the sample too much can destroy it. The lower resolution of dynamic mode can make it difficult to know how much the sample has been thinned.
Thus, the user typically watches the progression from slice to slice and looks for changes from slice to slice. This helps him know where the milling is taking place (where the beam “is hitting”), helps him see changes in his sample, and helps him follow progress towards the endpoint—where the user manually stops the mill, e.g., endpointing.
What is needed is a method of precisely and efficiently showing the operator changes in the slices so that real-time endpointing can be done in a better manner that produces less errors and higher productivity. The method lends itself further with other automated processes that increase throughput and reproducibility of TEM samples.